Field of the Invention
The present invention relates to printing position adjustment control that adjusts an image forming position on a sheet.
Description of the Related Art
A typical image forming apparatus includes a function of adjusting a printing position of an image (hereinafter, referred to as “printing position adjustment”) in order to print an image at an intentional position of a sheet. Adjusting the printing position makes it possible to align the printing position of an image on a front surface and the printing position of an image on a rear surface with high accuracy in both-surface printing to provide a high-quality printed matter. Further, for example, in a case of printing an image on a preprinted paper on which ruled lines and the like are previously printed, it is possible to provide a high-quality printed matter in which the ruled lines and the image are printed so as not to be overlapped with each other.
It is necessary to perform the printing position adjustment for each sheet on which the image is to be printed, because an expansion/contraction amount of a sheet is varied depending on a size, a basis weight (weight), quality, etc. of the sheet.
The printing position adjustment is commonly performed with use of an adjustment chart that is created by printing a mark on a sheet to be adjusted. The image forming apparatus detects a deviation amount of the printing position, based on, for example, information of a length from a reference position to the mark of the adjustment chart, and determines a correction condition to correct the printing position, based on a detection result. Further, the image forming apparatus corrects the printing position, based on the correction condition in a case where printing processing is performed with use of a sheet of the same type as the sheet to be adjusted.
It was known from an experiment that a dimension of the sheet is varied depending on a moisture content. For example, when an image is printed with use of sheets of the same types that are left under environment different in humidity, printing positions on the sheets are different from each other as a result. The sheet used for printing is typically sold while being wrapped by a wrapping paper to maintain sheet quality. For example, the wrapping paper has been subjected to a special treatment in order to prevent the sheet from absorbing moisture. In a case where the sheet is housed in a container of the image forming apparatus, an operator opens the wrapping paper and houses the sheet in the container. Therefore, the moisture content of the sheet may start to change after unwrapping.
In a case where the sheet taken out from the wrapping paper is housed in the container of the image forming apparatus that is placed in a room with low humidity, the moisture content of the sheet is decreased with time. Accordingly, the dimension of the sheet that has been housed in the container for a long term is reduced as compared with the dimension of the sheet immediately after unwrapping. Alternatively, depending on a type of the sheet, the dimension of the sheet that has been housed in the container for a long term may be increased as compared with the dimension of the sheet immediately after unwrapping.
An image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2005-221582 includes a timer, automatically prints a test sheet every time operation time of the image forming apparatus becomes a predetermined time, conveys the test sheet to a reading sensor, and adjusts a printing position, based on a reading result of a test image on the test sheet.
Various types of sheets are housed in the container of the image forming apparatus. For example, the sheets of the type called A4 and LTR are conveyable in any of the following directions: a direction in which a conveyance direction of the sheet conveyed by the image forming apparatus and a longitudinal direction of the sheet are orthogonal to each other (hereinafter, referred to as vertical placement), and a direction in which the conveyance direction and the longitudinal direction of the sheet are parallel to each other (hereinafter, referred to as horizontal placement).
The sheet has a known physical property that is called grain. This indicates a direction of fibers of the sheet. The fibers of the sheet are likely aligned in a direction in which pulp flows in a manufacturing process of the sheet. The sheet is manufactured by cutting a roll-shaped long sheet. Accordingly, the grain of the roll-shaped sheet is fixed in one direction. Depending on a cut direction when the long sheet is cut, however, two types of sheets, a sheet having the grain parallel to a long side (hereinafter, referred to as long grain) and a sheet having the grain parallel to a short side (hereinafter, referred to as short grain) are manufactured. In other words, the grain is a property determined by the manufacturing process of the sheet.
A sheet has a property of expansion/contraction caused by drying and absorption of moisture. Further, it is known that the direction and the degree of expansion/contraction are changed depending on the direction of the grain. In other words, the direction and the degree of expansion/contraction may be different in sheets having the vertical grain and the horizontal grain even in a case of the same type of sheets with the same size. For example, the conveyance direction of the sheet and the grain are parallel to each other when the A4 sheet of the horizontal grain is vertically placed, and the conveyance direction of the sheet and the grain are orthogonal to each other when the A4 sheet of the horizontal grain is horizontally placed. For example, the conveyance direction of the sheet and the grain are orthogonal to each other when the A4 sheet of the vertical grain is vertically placed, and the conveyance direction of the sheet and the grain are parallel to each other when the A4 sheet of the vertical grain is horizontally placed. Accordingly, there are two cases of the vertical grain and the horizontal grain even though sheets have the same A4 size and are placed vertically. In other words, even in the case of a predetermined type of A4 sheet, parameters for adjustment of the printing position may be different from each other depending on whether the A4 sheets are housed in vertical placement or in horizontal placement.